In absolute angular position encoders, actual position values can be transmitted, preferably serially, for example to a controller for further processing, in binary form at predefined time intervals. Since the operation of serial data transmission of all the bits of an actual value of this type that is encoded in binary form takes up a specific time duratio, the real actual value can vary sharply during said operation of the serial data transmission.
A first case of this type is presented when the shaft of the absolute angular position encoder temporarily rotates at a high rotational speed. It may occur, for example, because of an excessive speed that is intrinsically inadmissible. In such case, the factual position of the shaft may change by many distance units between the beginning and end of the serial, bitwise transmission of an actual position value that was detected previously. The subsequent, serially transmitted actual position value then has an absolute magnitude which has changed in an unexpectedly abrupt manner.
A further case of this type is presented when errors occur during the serial data transmission. In this case, high-value bits in the encoding of the actual position value can be falsified in the serial data stream. Such occurrence, for example, may be caused by electromagnetic irradiation. In such case, actual position values may have an absolute magnitude which, induced by the error, changes in an abrupt manner. If such abrupt changes in an actual angular position value are fed to a controller, for example, then both the identification of the rotational direction of the shaft of the angular position encoder, and the testing of the plausibility of the incoming actual position values, are made more difficult.
In normal operation, the direction of rotation of the shaft of the angular position encoder can be ascertained in a simple manner by determining the difference between successive actual values. If the sign of "current actual position value - previous actual position value" is positive, then the rotation is "forward", for example; if a negative sign occurs, then the rotation is "backward". However, a precondition for the functional capability of this method is that the two actual values in the above difference are always positioned so close to each other that such a pair in each case lies either "before" or "after" the so-called "zero point" of the shaft of the angular position encoder. As such, the absolute magnitudes of the two actual values must generally not deviate sharply from each other.
However, the rotational speed of the shaft, and the rapidness of the serial data transmission, must always be matched to each other in such a way that one of the two values of the pair is not acquired "before" and the other value acquired "after" passing over the zero point. In such a case, an erroneous direction of rotation would be determined. For instance, if the old actual value, lying "before" the zero point, is 3550, (corresponding for example to an actual position value of 355.0 degrees), and the new actual value, lying "after" the zero point, is 50 (corresponding for example to an actual position value of 5.0 degrees), then, the difference "current actual position value--previous actual position value", i.e. 50-3550, results in a negative sign. This occurs even though the shaft has been rotated forward over the zero point. On the other hand, actual position values which have been severely corrupted in terms of magnitude as a result of electromagnetic irradiation must also not be used for the difference formation. There is the risk in this case, that an erroneous rotational direction will be determined.
Actual position values which have changed sharply in an unexpected or error-induced manner also constitute a problem if these are checked for plausibility, for example in a controller. To this end, it is generally assumed that the change in the incoming actual position values takes place in an approximately quasi-continuous manner. It is then possible to check whether their absolute magnitude exceeds a predefined, maximum admissible rate of change. By this means, actual position values that are unusable, for example falsified because of errors during the serial data transmission, can be filtered out. On the other hand, such large jumps in the actual value can also occur unexpectedly when the matching between the speed of the serial data transmission and the rotational speed of the shaft of the angular position encoder is disturbed. This may occur as a result of a temporary, intrinsically inadmissible, excessive speed of rotation. Such large, faulty or unexpected jumps in the actual value can then be confused with admissible jumps in the actual value, which always occur during the acquisition of the angular position.
During the acquisition of the position of the shaft, the actual value specifically changes from the maximum value to the starting value or zero value, or vice versa, at the moment of traveling over the so-called zero point. With an encoder having, for example, a 0.1.degree. resolution of a complete revolution of the shaft being monitored, the binary coding of the actual value changes from a maximum value of 3599 (corresponding, for example, to an actual position value of 359.9 degrees) to 0 (corresponding, for example, to an actual position value of 0 degrees) when traveling over the zero point. Such jumps in the absolute magnitude of the actual position value are admissible only at the moment of passing over the zero point and must be identified as such. If, however, as described above, sporadically unexpected or error-induced large jumps in absolute magnitude occur, both the identification of the rotational direction of the shaft and the testing of the plausibility of incoming actual position values are hampered by this, for example in a controller carrying out the evaluation.
European Patent Application No. 0 479 525 A2 describes an absolute angular position encoder. In the event of a power failure of the external voltage supply, this supply is buffered by an internal voltage supply. The absolute angle of a rotating element is calculated using the output signals from two detecting coils that are arranged at an angle of 90 degrees to each other. The rotational direction is calculated by comparing the two output signals from the detecting coils. If a transition occurs in one output signal from `low` to `high`, then the status of the second output signal is checked. Depending on whether the latter is `low` or `high`, there is positive or negative rotational motion.
Japanese Patent Application No. 6 258 098 relates to an absolute angular position encoder, in which the absolute angular value, which is initially present in binary form, is output serially. For the purpose of angular measurement, use is made in particular of a single revolution counter and a multiple revolution counter.